This invention relates to a magnetic head control apparatus for controlling the seek operation of the magnetic head in a magnetic disc apparatus.
In a typical magnetic disc apparatus, after positioning of the magnetic head (hereinafter referred to as a head) in relation to the magnetic disc platter (hereinafter referred to as a magnetic disc) reading/writing of data is performed at a prescribed location on the disc. This kind of positioning operation is referred to as a normal seek operation.
The control system of the seek operation can be divided into positioning control in which the velocity of the head moving to the prescribed target track on the disc is controlled and the head is positioned accurately over that track, and also for transient control of the time after the switch from velocity control to position control.
As is shown in FIG. 1, a head 11 for recording/reproducing data and a head 12 for reproducing servo data are provided in relation to a disc 10. The servo data reproducing head 12 reproduces servo data (position data corresponding to the position of the track on the disc) prestored on the servo surface of disc 10, and extracts the necessary signals for the positioning of head 11. Both heads 11 and 12 are attached to and are transferred by carriage 13, which is driven by a voice coil motor 9, which in turn is driven by a current I from a drive circuit (not shown), and disc 10 is rotated by motor 14.
When, for example, as is shown in FIG. 2A, head 11 performs a seeking operation from track D, which is at point D on disc 10, to track A, which is at point A, a target velocity MV corresponding to the seek distance is set by a controller (not shown). The controller detects the track on which the head is positioned by counting the track pulses from a track detection circuit for each crossing of the center of a track by the head. Then, the controller calculates the distance (i.e., the number of tracks) between the detected track D and the target track A and calculates the target velocity MV based on this distance. The drive circuit supplies current I to voice coil motor 9 based on the target velocity MV set by the controller to transfer head 11 at the target velocity. In the initial period of the seek operation, the head 11 is accelerated by voice coil motor 9, as is illustrated by curve Vl in FIG. 2A, to the target velocity MV shown at E. When the head 11 is moved further and reaches point F, the target velocity MV calculated by the controller is decreased from value G and until it reaches zero at point A. Drive current I is supplied to voice coil motor 9, as shown in FIG. 2B, for moving head 11 in this way. In the seek operation in which head 11 moves from point B, shown in FIG. 2A, to point A, the head 11 moves first with the target velocity set at value H. Then, along with the transfer of the head 11, the target velocity MV is varied from value H to value C by the controller. At this time, when the head 11 starts at point B and moves to point A, head 11 is accelerated as shown by curve V2 in FIG. 2A until it reaches the target velocity value C. Then, head 11 is moved to point A in response to the target velocity MV shown by curve V3 of FIG. 2A. At this time a drive current I such as that shown in FIG. 2C is supplied to voice coil motor 9 from the drive circuit.
With this kind of velocity control in the seek operation, when the seek distance is large and the target velocity MV of the head increases, the transfer of the head necessitates a further revision in the target velocity high speed. The controller revises the target velocity synchronously with the track pulses produced with each movement of the head. When a relatively long seek operation such as from track D to track A is performed, the target velocity is increased so that the actual velocity of the head increases and the interval between track pulses decreases and, consequently, it is difficult for the controller to revise the target velocity synchronously with the track pulses, and at worst the seek accuracy is affected.